Polymer electrolyte fuel cells (PEFCs) have great potential as an environmentally friendly energy source. Fuel cells are electrochemical energy converters which feature in particular a high level of efficiency. Among the various types of fuel cells, PEFCs feature high power density and a low weight to power ratio. The PEFC uses as its electrolyte a polymer membrane.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells are attractive electrical power sources, due to their higher energy efficiency and environmental compatibility compared to the internal combustion engine. The most well-known fuel cells are those using a gaseous fuel (such as hydrogen) with a gaseous oxidant (usually pure oxygen or atmospheric oxygen), and those fuel cells using direct feed organic fuels such as methanol.
The polymer electrolyte membrane or proton exchange membrane (PEM) is an important aspect of any PEFC. PEMs are an excellent conductor of hydrogen ions. The most widely used materials to date consist of a fluorocarbon polymer backbone, similar to Teflon®, to which are attached sulfonic acid groups. The acid molecules are fixed to the polymer and cannot “leak” out, but the protons on these acid groups are free to migrate through the membrane. With the solid polymer electrolyte, electrolyte loss is not an issue with regard to stack life. The potential power generated by a fuel cell stack depends on the number and size of the individual fuel cells that comprise the stack and the surface area of the PEM.
In many fuel cells, the anode and/or cathode comprise a layer of electrically conductive, catalytically active particles (usually in a polymeric binder). A polymer electrolyte membrane is sandwiched between an anode and cathode, and the three components are sealed together to produce a single membrane electrode assembly (MEA). The anode and cathode are prepared by applying a small amount of a catalyst for example platinum (Pt) or ruthenium-platinum (Ru/Pt) in a polymeric binding to a surface that will be in contact with the PEM. Preparation of catalyst electrodes has traditionally been achieved by preparing an ink consisting of an electrocatalyst (either Pt or Ru/Pt), Nafion® polymer (5% wt. solution dispersed in lower alcohol). The ink is applied to porous carbon paper using a painting technique or directly depositing the ink upon the membrane surface or pressing it upon the membrane like a decal.
A MEA of a hydrogen fuel cell typically accepts hydrogen from a fuel gas stream that is consumed at the anode, yielding electrons to the anode and producing hydrogen ions which enter the electrolyte. The polymer electrolyte membrane allows only the hydrogen ions to pass through it to the cathode while the electrons must travel along an external circuit to the cathode thereby creating an electrical current. At the cathode, oxygen combined with electrons from the cathode and hydrogen ions from the electrolyte to produce water. The water does not dissolve in the electrolyte and is, instead, rejected from the back of the cathode into the oxidant gas stream.
For the last 30 years the industry standard for the PEM component of a hydrogen or methanol fuel cell has been membranes based on fluorine-containing polymers, for example the Nafion® material marketed by DuPont.
Nafion material is a perfluorinated sulfonic acid polymers having the following structure
which are often used as membrane material for fuel cells and which operate at temperatures close to ambient. Further, Nafion® polymer membranes are hydrated and they have a hydrogen ionic conductivity of about 10−2 S/cm or higher.
The Nafion® membranes display adequate proton conductivity, chemical resistance, and mechanical strength. Some of the membranes disadvantages are reduced conductivity at high temperatures (>80° C.), high methanol permeability in direct methanol fuel cells, relatively thick membranes and membrane dehydration at high elevated temperatures. Further, when Nafion® membranes are used at temperatures above 80° C., they thermally deform. This deformation of the membrane prevents the Nafion® membrane from coming into sufficient contact with the electrode, thereby reducing fuel cell performance. Additionally, there is a need to reduce the costs associated with such membranes.
Another limitation of Nafion® membranes occurs in applications in methanol fuel cells. Nafion® membranes are permeable to methanol. Methanol crossover is inversely proportional to membrane thickness. Direct transport of the fuel (i.e. methanol) across the membrane to the cathode results in losses in efficiency.
Increasing the membrane thickness results in decreased methanol crossover. However, thicker membranes result in Ohmic losses and decreased fuel cell performance.
Membranes that decrease the rate of methanol crossover would allow the use of higher concentrations of methanol-water feed mixtures, which would increase catalyst efficiency, direct methanol fuel cell power out put and potentially fuel utilization.
In general, increasing the operation temperature of fuel cells is advantageous for several reasons. Higher operating temperatures in methanol fuel cells decrease the carbon monoxide poisoning of the electrocatalyst. Higher temperatures increase reaction kinetics of hydrogen oxidation on the anode and oxygen reduction on the cathode. However, as the temperature is increased, it becomes more difficult to keep the membrane hydrated. Dehydration of membranes is exacerbated by relatively thick membranes. Dehydrated membranes lose ionic conductivity and result in poor contact between fuel cell components due to shrinkage of the membrane.
Therefore, improved performance of fuel cells would be achieved by reducing the thickness of membranes and improving the humidification state of solid PEMS since water molecules can promote proton transport and thin membranes can reduce ionic resistance and Ohmic losses.
Additionally, the contact between the membrane and electrode affects the efficiency of a fuel cell. Interfacial resistance between the membrane and electrode causes Ohmic loss thereby decreasing fuel cell efficiency.
Improving the membrane-electrode contact and continuity wherein the membrane and electrode are cast from a composition having the same or similar polymer electrolytes would improve the membrane-electrode interfacial resistance.
What is needed is a composition and nanocomposites from which improved polymer electrolyte membranes, electrodes and electrode casting solutions are made and that have improved performance at temps at about 80° C. and above. Operating at these temperatures results in enhanced diffusion rates and reaction kinetics for methanol oxidation, oxygen reduction, and CO desorbtion thereby producing a more efficient fuel cell.